The invention relates generally to procedures aiming at the mobility of a wireless terminal in a data transmission network, where data is transmitted in packages, i.e. cells. The invention particularly relates to an advantageous network structure for supporting wireless terminals and to a method for executing a handover in such a network structure.
The ATM (Asynchronous Transfer Mode) network is a data transmission system where data is transferred in digital form as 53-byte cells from one terminal to another through switches and rapid transfer connections therebetween. Each cell contains a 48-byte payload and a 5-byte header. In order to save space, the header information of each cell does not include complete address information describing the used data transfer route from the transmitting to the receiving device, but only information of the virtual path and channel where said data transfer connection is being carried. The switches or nodes of the network contain necessary routing information, on the basis whereof said identifiers of the virtual path and channel are interpreted as references to the respective node in succession.
It is to be expected that in the future the ATM network, which has so far been based mainly on cable connections, will also serve wireless terminals that are in contact with the network via radio base stations, i.e. access points. These wireless terminals may move with respect to the base stations and their coverage area, in which case the system must be able to execute a handover when necessary. A specific feature of each ATM connection is the contract between the terminal and the network as to the quality of service (QoS) required by the connection. This contract particularly covers the maximum length of the delays allowed in the connection and the highest allowed probability of cell loss. The agreed quality of service for the connection and its upkeep are important factors when making decisions as for the point of time of the handover and the new base station to be assigned for a given wireless terminal.
The PNNI protocol developed for the routing of a fixed ATM network defines how the switches of the ATM network can discover the network structure and transmit structural information to each other. The PNNI protocol also defines the routing method whichxe2x80x94on the basis of said structural data, the offered traffic and the required quality of service for the connection, as well as the available resources finds the most advantageous route to the new connection.
FIG. 1 illustrates an example of a network structure according to the PNNI protocol. According to the PNNI protocol the switches or nodes of the ATM network are grouped into peer groups, and one of the nodes in the group serves as the peer group leader. In FIG. 1, the peer group leaders are marked with black circles. A peer group is formed by such nodes that have a common ancestor in the PNNI address and routing hierarchy. The PNNI network structure is multi-layered: the peer group of one level forms, on the successive higher level, one logical node. The nodes of the lowest level are physical switches of the ATM network. In the PNNI protocol, each node only knows the structure of its own peer group and of those higher-level peer groups to which the peer group of said node belongs.
In a PNNI protocol each node maintains a database as for the structure of its own peer group and the connections of said peer group with other adjacent peer groups. For this purpose, the nodes transmit, at given intervals, information of their activity and their connections with other nodes via PTSE packages. Thus each node and peer group has real-time information of the network structure, so that for instance in the case of malfunction, the peer group is capable of changing the routing of connections past the damaged connection or node.
In FIG. 1, one peer group is formed for instance of the nodes A.1.1-A.1.5. On the next higher level, this peer group A.1 is represented by the logical node A.1. The nodes A.2.1-A.2.4 form another peer group A.2. On the higher level the logical nodes A.1-A.4, each of which represents a given lower-level peer group, form the peer group A. For the sake of clarity, all nodes of all peer groups are not individually specified in FIG. 1. Such lines that in the illustrations of the present applicationxe2x80x94particularly in FIGS. 1 and 2xe2x80x94are attached to the node at one end only represent connections directed to outside the part of the network included in the drawing. The PNNI protocol is described in more detail for instance in the publication xe2x80x9cPrivate Network-Network Interface Specification Version 1.0xe2x80x9d by The ATM Forum.
An object of the invention is to realise a network structure suited for the radio extension of an ATM network. Another object of the invention is to realise such a network structure which enables the adding of a radio extension supporting wireless terminals with minor changes to existing ATM networks.
Yet another object of the present invention is to introduce a method suited for the radio extension of an ATM network for executing an inter-switch handover.
These objects are achieved by advantageously providing only part of the ATM network switches with mobility-specific functions, by organising the access points of wireless terminals under the control of switches supporting wireless terminals, by providing logical connections between these wireless-specific switches in order to distribute control data concerning the wireless terminals, and by compiling the wireless-specific switches into a wireless-specific peer group structure.
The system according to the invention is characterised in that the structure formed by the wireless-specific switches and by the connections therebetween constitutes a parallel switch group structure with respect to the other parts of said data transmission network, in which case the wireless-specific switches belong both to a switch group of the ordinary switch group structure of said data transmission network and to a switch group of the switch group structure supporting wireless terminals.
The invention also relates to a method comprising the following steps, where
a) the switch controlling the first access point sends a handover request in a wireless-specific structure to the switch preceding it in the direction the connection came from,
b) in which case said preceding switch decides whether, it is the switch suited for controlling the handover:
b.1) if it is not, it sends the handover request in the wireless-specific structure to the preceding switch in the forward direction of the connection, whereafter the step b) is repeated,
b.2) if it is, it starts executing the handover to a second access point.
In the network structure according to the invention for realising a radio extension of an ATM network, only part of the ATM network switches are provided with mobility specific functions, in which case the adding of a radio extension does not require the renovation of the whole network. In between wireless-specific switches, there are provided logical connections for transmitting the control and management data concerning wireless terminals. The wireless-specific switches and the connections therebetween form a PNNI node structure parallel in relation to the rest of the ATM network, in which case the wireless-specific switches belong, apart from this PNNI node structure, also to the node structure of the regular ATM network. By means of this structure, the additions required by the support of wireless terminals in the PNNM protocol remain very slight, wherefore the wireless terminal connections do not differ from the connections between regular fixed terminals with respect to the switches of a regular ATM network.